Both barium sulphate and lithium metal phosphates are of great industrial importance:
Barium sulphate is used in particular for the preparation of photographic papers, varnishes and plastics and also in medical diagnostics and in medical plastics. Barium sulphate is a high-quality, pure-white inert, acid- and alkali-resistant filler which is extremely weatherproof.
Barium sulphate with a high so-called “brightness”, with a small specific surface and a low oil absorption number as well as good dispersibility is called “blanc fixe” and serves as white pigment and filler.
Particularly high-quality and finely dispersed “blanc fixe” types are also used in particular in cosmetics and sunscreens (WO 01/92157). Further details on “blanc fixe” are discussed for example by J. Hocken in Euro Coat, Lyon September 1997, pages 1 to 14.
Due to its high absorption for X-radiation and its high biological inertia, it also serves as a medical contrast medium.
Blanc fixe types with an average particle size of 0.5 to 1 μm and narrow grain-size spread, i.e. in particular with a monomodal particle-size distribution, have maximum light scatter effect and are therefore particularly suitable as white pigment or substitute for titanium white. (Machunsky, E. Winkler, J., Polymers Paint Colour Journal (1990) 180, 350-354).
Blanc fixe is usually prepared on an industrial scale by reacting caustic soda solution and sulphuric acid or sodium sulphate solutions with aqueous solutions of barium chloride or barium sulphide. The particle morphology is set by adjusting the reaction conditions such as temperature, concentration, pH, mixing and stirring speeds, addition of seed crystals or also organic additives.
In particular, organic additives have recently come into use to improve the particle morphology of barium sulphate (WO 01/58809).
There are strict requirements in respect of the purity of the starting solutions when preparing “blanc fixe”, in particular in respect of the presence of metals (e.g. transition metals and heavy metals) influencing the brightness, in order to obtain products with a high brightness.
According to WO 00/76919, barium sulphate can also be prepared from dissolved barium hydroxide with sulphuric acid, which, in addition to high raw material costs, is also economically unappealing due to the poor solubility of barium hydroxide.
More recently, lithium metal phosphate compounds, in particular lithium transition-metal phosphate compounds, have been widely used as cathode and anode materials in batteries (U.S. Pat. No. 5,910,382, WO 02/099913).
In addition to solid-state syntheses, wet-chemical processes for the preparation of such compounds are also used, as described for example in DE-10353266 or in WO02/083555. In such wet-chemical processes, the particle morphology of the produced lithium transition-metal phosphate can advantageously be particularly well controlled. Often in these processes i.a. lithium hydroxide is used as raw material, combined in one of the synthesis steps with an acid solution containing phosphoric acid and at least one transition-metal sulphate. However, the problem when preparing lithium transition-metal phosphates according to such wet-chemical processes was in particular the high loss of lithium ions in the so-called waste lye, the reuse of which in typical industrial cyclic processes, in particular in the working-up and recovery of the starting substances, is made difficult by high levels of foreign ions, in particular sulphate.
To date, only extremely laborious purification processes were known in order to make a reconstitution of the lithium-containing waste lye possible.